Tachometer interference
Seasoned HDF Member
Joined: Dec 2005
Posts: 19,828
Likes: 7,967
From: poway
Are you using metal core wires? That'll wreak havoc sometimes. Leaking plug wires could cause a problem also. Figured all you guys keep that tidy but I had to mention it.
On a quick search, seems the net all recommends a 10k 1/4 watt resistor. One guy says a rectifier diode on the tach feed wire. (?) Has anyone tried an ignition condenser on the coil negative/tach input?
My bike has the big FL dash with the Denso tach, uses a step motor and is smooth as silk plus my coil is stuffed up under the tank so I'm not gonna experiment.
But for y'all who have a coil on the left where its easy to get at, that'd be what I'd try next - plain old points style ignition condenser tied on at the feed wire to the tach.
On a quick search, seems the net all recommends a 10k 1/4 watt resistor. One guy says a rectifier diode on the tach feed wire. (?) Has anyone tried an ignition condenser on the coil negative/tach input?
My bike has the big FL dash with the Denso tach, uses a step motor and is smooth as silk plus my coil is stuffed up under the tank so I'm not gonna experiment.
But for y'all who have a coil on the left where its easy to get at, that'd be what I'd try next - plain old points style ignition condenser tied on at the feed wire to the tach.
I'll have to check the resistance I used but I doubt it will make any difference in my case. My problem sounds the same as OPs. Between 2000 to 3000 (even a little above), the needle reads 2 different RPMs. It will stay at say 2400 for maybe a second then move to 2700 for a second. I tried filter caps but did not use a condenser. The videos that I saw the tack needle would go crazy.
The same ignition with a better tach did not have this issue. The main problem I had was the bike would kill tach from vibration. I'd be riding down the road and the tack needle would fall off..
One thing to note is that in my case I was using a Dynatek 2KI ignition. It has a tach output that swings 12 volts. The guys that seem to have success in the some of the videos I saw were connecting directly off of the coil primary.. Primary voltages can be a couple hundred volts. I can see where adding a series resistor there would help..
The diode thing is for getting a single fire ignition to read the correct RPMs.. Most of the cheap tachs are looking for 1 pulse per rev. Single fire needs diodes to wire OR the 2 single fire signals.
BTW the Denso tachs (Japanese) worked the best and lasted the longest.
Seasoned HDF Member
Joined: Dec 2005
Posts: 19,828
Likes: 7,967
From: poway
BTW, Rode the bike that has the funky tach to lunch yesterday..
The needle pretty much wavers a little all the time but at 2400 rpm it jumps to 2700 and at 2700 it jumps to 3000. Most off the time it's got the small waver.. Say 50 RPM..
The needle pretty much wavers a little all the time but at 2400 rpm it jumps to 2700 and at 2700 it jumps to 3000. Most off the time it's got the small waver.. Say 50 RPM..
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